1. Field of the Invention
This invention relates generally to devices used for detection of contaminations deposited on optical elements. More specifically it relates to an improvement made on the applicant's prior U.S. Pat. No. 4,808,813 granted Feb. 28, 1989 entitled "Self Contained Surface Contamination Sensor for Detecting External Particulate and Surface Discontinuities".
Prior Art: In the applicant's prior patent, a sensor is disclosed that uses two diodes. Contamination, either in the form of particulate matter (for example: dust and non-wetting liquid) or surface discontinuities, such as a smooth film or cratering, is collected on an exposed glass plate. Illumination at an angle incident with respect to a surface of the glass plate causes the particulate matter to scatter the light beam from one of the diodes. The light beam from the other diode illuminates the inside volume of the glass plate, causing light to scatter from the surface discontinuities. Either source of light scattering is detected by an optically sensitive detector positioned below the glass plate. A bandpass filter between the glass plate and the detector rejects spurious radiation. Although the device accomplishes its intended task of detecting external particulate and surface discontinuities, it does not, however, distinguish which type of contamination has been deposited, so that the device is accordingly in need of further improvement.